In manufacturing semiconductor devices, as higher density and higher integration degree are required to the devices, multi-layered wiring structures are being increasingly used for circuitry. Under the circumstances, embedding techniques for electrical connection between layers have become important, for example, at contact holes used as connection portions between an Si substrate or poly-crystalline silicon layer on the lower side and wiring layers on the upper side, and at via-holes used as connection portions between upper and lower wiring layers.
In general, a metal, such as Al or W, or an alloy made mainly of these materials is used as a material for filling such contact holes and via-holes. In this case, it is necessary to form good contact between the metal or alloy and an underlayer, such as an Si substrate or poly-crystalline silicon layer. For this reason, before filling a filler material, a Ti film is formed on the inner surface of the holes and is caused to react with the underlying Si to thereby form a titanium silicide (TiSi) film as a contact layer. Then, a TiN film is further formed thereon as a barrier metal layer for the filler material.
In order to form Ti films or TiN films of this kind, chemical vapor deposition (CVD) is utilized, because this method can suppress increase in the electric resistance, provide the films with good quality, and attain high step coverage, even where devices are miniaturized and highly integrated.
Where a Ti film and a TiN film are formed by CVD, TiCl4 is used as a film formation gas, and thus Cl2 and HCl are generated as reaction products. When the TiN film is formed after the Ti film is formed, these reaction products act to etch the Ti film. This etching action brings about a problem such that the TiN film peels off the Ti film due to an insufficient adhesion degree between the Ti film and the upper layer or TiN film, and a thermal stress or the like applied when a filler metal film is formed.
Conventionally, in order to solve this problem, after the Ti film is formed, NH3 gas is supplied to nitride the Ti film, and then the TiN film is formed. According to this method, the Ti film is nitrided and is thereby prevented from being etched by Cl2 and HCl, so no film peeling is caused on the Ti film.
In recent years, in order to increase the operational speed of devices, there is a case where another metal silicide, such as cobalt silicide (CoSi2), is formed, in place of titanium silicide (TiSi), as a contact layer at the interface between the underlying Si and Ti film to be formed. This is adopted, because such a metal silicide provides the underlayer with a better contact property for the Ti film. For example, Patent Document 1 proposes a method for forming a Ti/TiN film on a cobalt silicide film disposed on the bottom of a contact hole. Patent Document 1 describes a method for forming a Ti film on a cleaned surface of the cobalt silicide film to form a good interface. Further, in recent years, as a metal silicide effective for logic contact, nickel silicide (NiSi or the like) has attracted attention. However, nickel silicide causes a phase transition from NiSi to a higher resistivity phase (Ni5Si2 or Ni3Si) when exceeding 500° C. Accordingly, where a Ti/TiN film is formed on a nickel silicide film at a high temperature of 500° C. or more, the nickel silicide film is changed from NiSi to a higher resistivity phase and increases the resistivity, thereby rendering a higher contact resistance. In order to prevent this problem, it is necessary to perform the Ti film formation, Ti film nitriding process, and TiN film formation at a film formation temperature of 500° C. or less, where the Ti/TiN film is formed on the nickel silicide film. For this reason, the Ti film formation, Ti film nitriding process, and TiN film formation have required the film formation to be performed at a lower temperature of 500° C. or less. Further, in recent years, due to miniaturization of devices, impurity diffusion layers formed in Si substrates have become thinner. This makes it more important to prevent the impurity diffusion layers from causing re-diffusion due to high temperature processes. Accordingly, it is also required to perform film formation at a lower temperature and to decrease the film thickness, where a Ti/TiN film is formed on an Si substrate.
However, Patent Document 1 described above pays no attention to such low temperature film formation. Where film formation is performed at a low temperature of 500° C. or less for the Ti film formation, Ti film nitriding process, and TiN film formation, there are differences described below, as compared to a case where the film formation is performed at a relatively high temperature of 600° C. or more, as conventionally used. Specifically, when the nitriding process is performed after the Ti film formation, the Ti film is not sufficiently nitrided. If the TiN film formation is performed after such insufficient nitridation, those portions of the Ti film (on the interface side adjacent to the underlayer) which have not been nitrided are etched by reaction products derived from the film formation gas, as described above. Consequently, the TiN film may peel off the Ti film due to an insufficient adhesion degree between the Ti film and the upper layer or TiN film, and a thermal stress or the like applied when a filler metal film is formed. Furthermore, even where film formation is performed at a low temperature of 500° C. or less, the contact resistance may become higher.
[Patent Document 1] Jpn. Pat. Appln. KOKAI Publication No. 2003-59861